Some focused ion beam (FIB) columns are intended for use with ion sources that emit multiple ion species. In order to select only one of these ion species for the beam to be focused onto a substrate, the FIB column will typically include a mass filter. One type of mass filter, a “Wien filter,” uses crossed electric and magnetic fields (ExB) to deflect unwanted ion species off-axis, thereby causing them to strike a mass-separation aperture. This type of filter is also referred to as an “ExB filter.” The relative strengths of the electric and magnetic fields are set so that the desired ion species will pass through the mass filter undeflected, then through the mass-separation aperture, and will finally be focused on the substrate surface.
Ions pass through the Wien filter within a “physical aperture,” that is, the area enclosed by the electric and magnetic pole faces. Ideally, the magnitudes of the magnetic field and the electric field would have the same ratio along the entire beam axis passing through the mass filter. In general, in the prior art, various electrode and pole piece configurations have been employed to achieve this field-matching goal, but maintaining the proper field ratio has been problematic near the end caps which terminate both the electric- and magnetic-fields at the entrance and exit to the mass filters. Typically, the end caps have small apertures through which the ion beams enter and exit the mass filter. Smoothly terminating the electric field without either E-field reversals or extra E-field peaks within the apertures is relatively easy with any reasonable electrical conductivity for the end cap metal. Conversely, since magnetic materials have permeabilities which are proportionately much lower than electrical conductivities (compared with air or vacuum), it has been found to be much harder to terminate the B-fields without negative overshoots or extra B-field peaks within the apertures.
What is needed is an improved method and apparatus allowing the adjustment of the B-field distributions within and near to the entrance and exit apertures of an ExB mass filter to obtain better matching between the E-field and B-field distributions over the entire axial length of the mass filter, including through the entrance and exit apertures. It would desirable for such an adjustment of the B-field to the E-field to be effected purely mechanically, without the need for electromagnet power supplies and electromagnetic coils added to the ExB mass filter for B-field adjustment.